InAs Quantum Dots Self-Organize on Microdisks

A self-organization process identified by scientists at Stanford University’s Solid State and Photonics Laboratory in California promises to facilitate the fabrication of devices in which quantum dots are coupled with microcavities, such as for proposed quantum information processing applications. They have found that InAs quantum dots formed on a prepatterned GaAs microdisk spatially align themselves in the cavity mode along the edge, which they attribute to preferential surface diffusion of adsorbed indium atoms in the structure during regrowth.

Quantum dots self-organize in the cavity mode along the edge of a prepatterned microdisk. The scanning electron micrograph shows a 4-μm-diameter, 50-nm-thick microdisk before regrowth. Images courtesy of Glenn S. Solomon, Stanford University.In their work, the researchers produced 50-nm-thick disks of GaAs 3 to 50 μm in diameter by molecular beam epitaxy and chemical etching. They then subjected the undercut disks to a regrowth cycle in the molecular beam epitaxy chamber, in which they deposited another 50-nm-thick layer of GaAs, 1.9 monolayers of InAs and a 100-nm-thick GaAs cap.

Atomic force microscopy of the disk after deposition of InAs quantum dots and before GaAs regrowth reveals the dots aligned along the disk edge.Scanning electron microscopy and atomic force microscopy revealed that InAs quantum dots formed only at the edge of the disks, in the narrow whispering gallery mode region of the structures. The linear density of the dots was proportional to the disk’s diameter.

To verify that the quantum dots were optically active, the investigators collected photoluminescence measurements of various disks, using a 1-μW, 670-nm CW laser diode as an excitation source. The resulting 890- to 940-nm emission from the edge was nearly continuous in the large disks with high densities of quantum dots, and was isolated to one to three spots in the smallest disks.